chore: fix gocritic lint issues

[alexandear]

The PR fixes the following issues:

❯ golangci-lint run
decode_hooks.go:281:2: singleCaseSwitch: should rewrite switch statement to if statement (gocritic)
        switch t {
        ^
mapstructure.go:1026:11: elseif: can replace 'else {if cond {}}' with 'else if cond {}' (gocritic)
                        } else {
                               ^
mapstructure.go:786:3: ifElseChain: rewrite if-else to switch statement (gocritic)
                if err == nil {
                ^
mapstructure.go:1010:7: wrapperFunc: suggestion: strings.Contains(tagValue[index+1:], "omitempty") (gocritic)
                        if strings.Index(tagValue[index+1:], "omitempty") != -1 && isEmptyValue(v) {
                           ^
mapstructure.go:1027:8: wrapperFunc: suggestion: strings.Contains(tagValue[index+1:], "remain") (gocritic)
                                if strings.Index(tagValue[index+1:], "remain") != -1 {
                                   ^
mapstructure_bugs_test.go:517:5: assignOp: replace `nanos = nanos % 1000000000` with `nanos %= 1000000000` (gocritic)
                                nanos = nanos % 1000000000
                                ^
mapstructure_examples_test.go:321:4: singleCaseSwitch: should rewrite switch statement to if statement (gocritic)
                        switch f.Kind() {
                        ^